The present invention relates to host cells which contain at least two functional recombinant polypeptides, at least one of which is bound to a support, preferably in each case in different cell compartments, e.g. cytosol, cytoplasmic membrane, periplasm and outer membrane, and also to methods for preparing said host cells. The cells of the invention are particularly suited as bioreactors for carrying out enzymatic reaction cascades for which compartmentalization of individual enzymes is advantageous or necessary.
The use of living cells as xe2x80x9cenzyme reactorsxe2x80x9d for preparing biological substances, e.g. polyhydroxy fatty acids, is of great importance for the biotechnological industry. To this end, it is common to introduce foreign genes into a host cell and to express said genes, in order to obtain in this manner a host cell with recombinant enzymes, which is capable of synthesizing a desired product. A disadvantage of known methods, however, was that the enzymes generated in the host cells by expression of the foreign genes were not stable, had too little activity or were present in too small an amount. Particular difficulties also stepped when carrying out multistage reactions in which substrates or products of one stage may have an adverse effect on other stages.
It was an object of the present invention to eliminate at least partially the problems of the prior art and to provide host cells which are capable of presenting functional recombinant polypeptides in a stable form and, in particular, of carrying out multistage enzyme reactions.
The object is achieved by providing a host cell comprising at least two functional recombinant polypeptides at least one of which is bound to a support.
Surprisingly, it was found that support-bound recombinant expression of heterologous polypeptides preferably in different compartments of a cell, e.g. of Gram-negative bacterial cells or of eukaryotic cells, leads to stable presentation of the heterologous polypeptides in a functional (i.e. immunologically or/and biologically, e.g. enzymatically, active) form. If the host cell is a Gram-negative bacterial cell, the cell compartments may preferably be selected from the cytosol, the cytoplasmic membrane (outside and inside), the periplasmic space and the outer membrane (outside and inside). If the host cell is a eukaryotic cell, said compartments may preferably be selected from the cytosol, the cytoplasmic membrane (outside and inside) and cell organelles such as, for example, Golgi, lysosomes, mitochondria, chloroplasts, vacuoles or endoplasmic reticulum.
The functional recombinant proteins present in the host cell are preferably cooperative, i.e. they fulfill a common immunological or/and biological function, for example as enzymes in a multistage reaction cascade.
At least one, preferably a plurality, of the functional recombinant polypeptides is bound to a support, for example in the form of fusion polypeptides, and contain at least one functional domain and at least one support domain. Preferred forms of support-bound polypeptides are S-layer structures (fusion polypeptides with S-layer support domains), membrane-bound polypeptides (fusion polypeptides with membrane-integrated support domains) or/and components of recombinant phage structures. If it is desired to export the functional polypeptides from the cytosol to other cell compartments, said polypeptides are expressed together with suitable targeting domains which facilitate export to the cell compartment desired in each case. Examples of targeting domains are signalpeptide or/and helper sequences which facilitate passage through the membranes.
In a preferred embodiment of the invention, at least one of the support-bound polypeptides is present as recombinant S-layer structure. S-layers are crystalline bacterial cell surface proteins which are composed of identical self-assembled units. Genetic data and sequence information for various S-layer genes from microorganisms can be formed, for example, in Peyret et al. (Mol. Microbiol. 9 (1993), 97-109). These data are expressly incorporated by reference.
Preferred S-layer genes are the B.stearothermophilus PV72 genes sbsA and sbsB. The sequences of these genes can be found, for example, in the international patent application PCT/EP97/00432 which also discloses production of a recombinant S-layer fusion protein in the cytoplasm of Gram-negative host cells. The international patent application PCT/EP98/04723 in turn describes production of a recombinant S-layer protein in various compartments of Gram-negative bacteria cells or eukaryotic cells. Regarding the construction of recombinant S-layer genes and production of suitable expression constructs, these two said international applications are expressly referred to. However, no indication of coexpression of two different functional recombinant polypeptides is found there.
Surprisingly, it was found that it is possible to co-express simultaneously or/and sequentially a plurality of recombinant S-layer proteins, where appropriate in combination with further heterologous proteins, for example in various compartments of host cells, in particular of Gram-negative bacterial cells and eukaryotic cells.
The nucleotide sequence of the gene coding for the mature SbsA protein is indicated from position 91-3684 in SEQ ID No. 1. The corresponding amino acid sequence is depicted in SEQ ID No. 2. The nucleotide sequence of the gene coding for the mature SbsB protein is indicated from position 94-2763 in SEQ ID No. 3. The corresponding amino acid sequence is depicted in SEQ ID No. 4.
In a first preferred embodiment (sbsA), the nucleic acid coding for the support domain of a functional peptide is selected from
(i) a nucleic acid which comprises the nucleotide sequence from position 91 to 3684 shown in SEQ ID No. 1,
(ii) a nucleic acid which comprises a nucleotide sequence corresponding to the nucleic acid from (i) within the framework of the degeneracy of the genetic code, and
(iii) a nucleic acid which comprises a nucleotide sequence hybridizing with the nucleic acids from (i) or/and (ii) under stringent conditions.
In a second preferred embodiment (sbsB), the nucleic. acid coding for the support domain of a functional peptide is selected from
(i) a nucleic acid which comprises the nucleotide sequence from position 94 to 2763 shown in SEQ ID No. 3,
(ii) a nucleic acid which comprises a nucleotide sequence corresponding to the nucleic acid from (i) within the framework of the degeneracy of the genetic code, and
(iii) a nucleic acid which comprises a nucleotide sequence hybridizing with the nucleic acids from (i) or/and (ii) under stringent conditions.
xe2x80x9cStringent hybridizationxe2x80x9d in accordance with the present invention means that hybridization still occurs even after washing at 55xc2x0 C., preferably 60xc2x0 C., in an aqueous low-salt buffer (e.g. 0.2xc3x97SSC) (see also Sambrook et al. (1989), Molecular Cloning. A Laboratory Manual).
Preferred sites for inserting peptide- or polypeptide-coding sequences into the sbsA gene are regions between positions 200-3600 of the nucleotide sequence shown in SEQ ID No. 1. Particularly preferred insertion sites are the NruI cleavage site at position 585, the PvuII cleavage site at position 881, the SnaB I cleavage site at position 920, the PvuII cleavage site at position 2507 and the PvuII cleavage site at position 2652 (PCT/EP 97/00 432). Further preferred insertion sites are positions 562, 1087, 1813, 1947, 2295, 2652, 3046, 3484 and 3594. The positions indicated in each case refer to the first nucleotide of the insertion.
Preferred sites of insertion into the sbsB gene are regions between positions 200-2600 of the nucleotide sequence shown in SEQ ID No. 3. Particularly preferred insertion sites are positions 410 (codon 136), 484 (codon 161/162) and 1583 (codon 528/529) (PCT/EP 97/00432). Further preferred insertion sites are positions 598, 1012, 1435, 1808 and 2301, the position indicated in each case referring to the first nucleotide of the insertion.
Alternatively or additionally, it is also possible to produce support-bound polypeptides in a different form, for example as components of recombinant phage structures, e.g. xc3x8X174 or xc3x8CH1.
Yet another possibility of producing support-bound polypeptides is the synthesis as fusion polypeptides with membrane integration domains so that the functional polypeptides are located at the locations desired in each case (outside or inside of a membrane chosen in each case).
The support sequence used for integration into the outer membrane of prokaryotic Gram-negative host cells may be the C-terminal domain of IgA protease from Neisseria or Haemophilus (Klauser et al., J. Mol. Bio. 23.4 (1993), 579-593). Further suitable support domain sequences are OmpA or LamB sequences or parts thereof ( . . . ).
For integration into the cytoplasmic membrane of Gram-negative prokaryotic host cells, preference is given to using a hydrophobic nonlytical membrane-integrating protein domain which has an xcex1-helical structure. Examples of DNA sequences coding for such a membrane-integrating protein domain are described in the European patent 0 516 655.
For secretion into the periplasm of Gram-negative prokaryotic cells it is possible to use, for example, the malE signal peptide sequence. Other sequences which cause secretion into the periplasm are described, for example, in Blondel and Bedouelle (Eur. J. Biochem 193 (1990), 325-330; Adip-Conquy et al. (Protein Eng. 8 (1995), 859-863); Weller et al (Eur. J. Biochem. 236 (1996), 34-39) and Dubreuil et al. (FEMS Immunol. Med. Microbiol. 13 (1996), 317-323).
Known signal peptides for expression in the cytoplasmic membrane or in organelles of eukaryotic cells are the N-terminal transit peptide of plastocyanin for transport into chloroplasts (Weisbeek et al., J. Cell. Sci. Suppl. 11 (1989), 199-223), mitochondrial signal peptides for transport into mitochondria (Skerjanc, Biochem. Cell. Biol. 68 (1990), 9-16), targeting sequences for transport into vacuoles (Vitale and Chrispeels, Bioessays 14 (1992), 151-160), targeting sequences for the cell membrane, cytoplasm and Golgi apparatus (Stanley, Mol. Membr. Biol. 13 (1996), 19-27), retention signals for the endoplasmic reticulum (Lencer et al., J. Cell. Biol. 131 (1995),:951-962) and transfer sequences for the Golgi apparatus or the plasma membrane (Rambourg et al., Anat. Rec. 245 (1996), 447-458).
It is possible for the DNA sequence coding for the foreign polypeptide to contain, in addition to the segment coding for the signal peptide, one or more further segments coding for further protein domains. Such a segment may preferably be located between the segment coding for the signal peptide and the segment coding for the foreign polypeptide. This segment preferably codes for a secretory polypeptide from Gram-negative bacterial or eukaryotic organisms or a part thereof. A preferred example of such a nucleic acid segment is the malE gene which encodes maltose binding protein.
The foreign polypeptides are preferably selected from DNA-binding epitopes, antigenic, allergenic or immunogenic epitopes, metal-binding epitopes, stretavidin, enzymes, cytokines or antibody-binding proteins.
A preferred example is stretavidin which is suitable for docking biotinylated reagents, e.g. after integration into the outer membrane. Another preferred example is antigenic, allergenic or immunogenic epitopes, for example epitopes from pathogenic microorganisms such as, for example, bacteria, fungi, parasites etc., and viruses, or epitopes from plants or epitopes against endogenous substances, e.g. cytokines, and also against toxins, in particular endotoxins. Particularly preferred examples of immunogenic epitopes are epitopes from viruses, for example from herpesviruses such as, e.g., herpesvirus 1, e.g. glykoprotein xcex94, herpesvirus 6 or pseudorabies virus (Lomniczi et al., J. Virol. 49 (1984), 970-979), in particular epitopes from the gB, gC or/and gD genes, epitopes from foot-and-mouth disease virus (FMDV), in particular epitopes from the gene segments coding for VP1, VP2 or/and VP3, epitopes from flaviviruses or epitopes from filoviruses such as, for example, Ebola, Marburg or Lassa virus. The immunogenic epitopes may be selected such that they promote generation of an antibody-mediated immune reaction or/and promote generation of a cellular immune reaction, for example by stimulation of T cells. Examples of suitable allergenic epitopes are birch pollen allergens, e.g. Bet v I (Ebner et al., J. Immunol. 150 (1993) 1047-1054). Particular preference is furthermore given to antigenic epitopes which are capable of binding and filtering out endogenous or exogenous substances such as, for example, cytokines or toxins from serum or other body fluids. Epitopes of this kind may include components of cytokine or toxin receptors or of antibodies against cytokines or toxins.
Modified foreign polypeptides, for example S-layer proteins; which have immunogenic or/and antigenic epitopes with glycosylation sites, are preferably produced in eukaryotic host cells in which glycosylation is possible. In this connection it is also possible to glycosylate the natural S-layer sequences. Examples of potential N-glycosylation sites in the S-layer gene sbsA are amino acid positions 26, 285, 343, 384, 387, 388, 418, 421, 483, 653, 675, 902, 924, 1048, 1058, 1118, 1154 and 1161. A potential N-glycosylation in the sbsB gene can occur in positions 155, 184, 213, 302, 303, 400, 463, 606, 755 and 915. Further possible modifications of the sbsA gene include amidation, phosphorylation by casein kinase II, N-myristoylation and phosphorylation by protein kinase C. Further possible modifications of the sbsB gene include phosphorylation by CAMP and cGMP-dependent protein kinase, phosphorylation by casein kinase II, N-myristoylation, phosphorylation by protein kinase C and attachment to a fibronectin receptor (via sequence RGD).
Likewise preferred foreign polypeptides are cytokines such as, for example interleukines, interferons or tumor necrosis factors. These molecules may be used, for example, in combination with immunogenic epitopes for the production of vaccines. In addition, antibody-binding proteins such as, for example, protein A or protein G, or DNA- or/and metal-binding epitopes such as, for example, leucine zipper, zinc finger, etc are also preferred.
The recombinant polypeptides are particularly preferably enzymes, in particular enzymes which catalyze a multistage enzymatic reaction. Specific examples are enzymes for the synthesis of polyhydroxyalkanoates, e.g. polyhydroxybutyric acid synthase (Lubitz and Resch, DE 44 171 69 A1; Slater, S. C., Voige W. H., Dennis, A. E. J. Bacteriol. (1998), 170:4431).
The present invention still further relates to recombinant bacterial ghosts obtainable from a Gram-negative host cell of the invention, which has at least two functional recombinant polypeptides bound to a support.
The preparation of suitable xe2x80x9cbacterial ghostsxe2x80x9d is described, for example, in the international patent application PCT/EP91/00967 which is hereby incorporated by reference. Said patent application discloses modified bacteria obtainable by transformation of a Gram-negative bacterium with the gene of a lytic membrane protein from bacteriophages, with the gene of a lytic toxin-releasing protein or with genes which contain part sequences thereof coding for lytic proteins, culturing of the bacterium, expression of this lysis gene and isolation of the resulting bacterial ghost from the culture medium.
As described in the European patent 0 516 655, the membrane of these bacteria may have bound to it a recombinant protein which is obtainable by expression of a recombinant DNA in these Gram-negative bacteria. This recombinant DNA comprises a first DNA sequence which codes for a hydrophobic, nonlytic membrane-integrating protein domain which has an xcex1-helical structure and consists of 14-20 amino acids which may be flanked N- and C-terminally by in each case 2-30 amino acids of any kind. This first DNA sequence is operatively linked to a second DNA sequence which codes for a desired recombinant protein. Furthermore, the Gram-negative bacterium contains a third DNA sequence which is controlled separately from the first and second DNA sequences and codes for a lytic membrane protein from bacteriophages or a lytic toxin-releasing protein or for lytic parts thereof. Expression and lysis of recombinant Gram-negative bacteria of this type produce so-called xe2x80x9cbacterial ghostsxe2x80x9d which contain an intact surface structure with immunogenic epitopes bound to the surface.
The preparation of host cells of the invention is preferably carried out by a method in which
(a) a host cell which has been transformed with at least two nucleic acids coding for recombinant polypeptides is provided, at least one of the nucleic acids being linked to a sequence coding for a support domain, in order to facilitate expression of the recombinant polypeptide in a support-bound form,
(b) the host cell is cultured under conditions which lead to expression of the nucleic acids and to a generation of the polypeptides encoded thereby in a functional form.
Furthermore, the nucleic acids coding for the recombinant polypeptides are preferably operatively linked to sequences which provide for localization of the recombinant polypeptides in each case in different compartments of the host cell.
When expressing the recombinant proteins in support-bound form as modified S-layers, it is in addition also possible to express genes in the cell which code for an unmodified S-layer protein. In this case, it is possible for the modified S-layer proteins to form an S-layer structure which is compatible with the unmodified S-layer proteins. An example of this embodiment of the method of the invention is an E.coli cell transformed with four S-layer genes, two of which are natural sbsA or sbsB genes and the other two are recombinant sbsA or sbsB genes.
The nucleic acids coding for the recombinant polypeptides are preferably located on recombinant vectors which contain at least one copy of the nucleic acid. The vectors used may be conventional prokaryotic or eukaryotic chromosomal or extrachromosomal vectors. Examples of such vectors are described in Sambrook et al., supra. The vectors contain the nucleic acids coding for the recombinant polypeptides, operatively linked to an expression control sequence active in the particular host cell. The expression control sequence particularly preferably comprises a controllable promoter. Examples of suitable prokaryotic promoters are the tac, lac, trp and xcex promoters. Examples of suitable eukaryotic promoters are the SV40, CMV and metallothionein promoters. The at least two nucleic acids coding for heterologous polypeptides are particularly preferably expressed by using two different controllable promoters, for example two different temperature-sensitive xcex promoters, as already described.
The recombinant host cells (living host cells or ghosts) are suitable for a multiplicity of applications. A use as vaccine or adjuvant is preferred, and in this case recombinant polypeptides are used which comprise immunogenic epitopes of pathogenic and/or endogenous immunostimulating polypeptides such as, for example, cytokines.
Particular preference is given to using the host cells or/and bacterial ghosts of the invention as enzyme reactors.
The present invention is furthermore illustrated by the following examples and figures, in which
SEQ ID NO. 1 shows the complete nucleotide sequence of the coding segment of the B.stearothermophilus S-layer gene sbsA;
SEQ ID NO. 2 shows the amino acid sequence derived therefrom;
SEQ ID NO. 3 shows the complete nucleotide sequence of the coding segment of the B.stearothermophilus S-layer gene sbsB;
SEQ ID NO. 4 shows the amino acid sequence derived therefrom.
FIG. 1 shows a diagrammatic representation of the possibilities for locating heterologous polypeptides in different compartments of a Gram-negative bacterial cell.
(a) A Gram-negative bacterial cell is composed of the cytoplasm (cy), the inner membrane (im), the periplasm (pp) and the outer membrane (om).
(b) Heterologous polypeptides can be exported to the periplasm by linkage with suitable targeting sequences (pe).
(c) Heterologous polypeptides can be anchored on the inside of the inner membrane (mae).
(d) In the periplasm, heterologous polypeptides can be immobilized in the form of recombinant S-layers (sie).
(e) Not only one but a plurality of species of recombinant heterologous polypeptides can be immobilized as S-layers in the periplasm.
FIG. 2 shows the diagrammatic representation of a recombinant bacterial cell of the invention, which contains various heterologous polypeptides (e.g. enzymes) in different compartments.